On railroad networks, such as the British Rail network, over-ground and underground signal heads are used to notify train drivers of whether a route is clear to continue their journey, or if a track is closed. Different colored signals, such as between two and four “aspects” (coloured light signals) can be used to convey different messages. As an example, red light (the stop aspect) signifies danger, telling the driver to stop, while green light informs a driver that a line is clear. Optionally, one or two yellow lights can also be used to caution drivers that they may need to stop at a signal further down the line.
Traditionally, coloured light signals have been produced using an incandescent white light source in combination with coloured lenses. The use of such a source has led to at least two problems. At long distances, light transmitted from red and green lenses can blend to give the appearance of white light. This is referred to as “colour blending” or “colour bleeding.” Further, incandescent lamps typically have short lifetimes, requiring replacement every six to twelve months. Failure of the filament results in the lamp instantly turning off Though safety procedures are in place to avoid danger from signal failure, maintenance can cause major disruption to rail services such as diversions, delays and cancellations, often at short notice and with a high associated cost.
Lighting systems comprising light-emitting diodes (LEDs) emitting at multiple wavelengths have been introduced to overcome the problem of colour blending and the short life-span of incandescent lamps for railway signaling applications. Using red and green LED signal heads, each comprising LEDs emitting across a wavelength gradient, colour blending can be alleviated. However, complex circuitry is required. Further, the cost associated with the manufacture and testing of these LED systems is high.
LEDs are traditionally made from inorganic semiconductors, which emit at a specific wavelength, e.g. AlGaInP (red), GaP (green), ZnSe (blue). Other forms of solid state LED lighting include organic light-emitting diodes (OLEDs), wherein the emissive layer is a conjugated organic molecule wherein delocalised π electrons are able to conduct through the material, and polymer light emitting diodes (PLEDs), in which the organic molecule is a polymer. Advantages of LEDs over traditional incandescent lighting include superior longevity, lower energy consumption resulting from less energy loss as heat, superior robustness, durability and reliability, and faster switching times. This is advantageous for use in transport networks, where downtime associated with maintenance work can be costly. However, solid-state lighting (SSL) is expensive and it requires different materials to emit at different wavelengths. Further, long-red emission (beyond 660 nm) is difficult to achieve using LEDs. Consequently, signalling systems requiring a range of emission wavelengths to display one colour are unfavourable using LEDs.
Strategies to tune the emission of a single wavelength of LEDs include the use of phosphors. LEDs emitting in the UV or blue region of the electromagnetic (EM) spectrum, which are generally cheap and readily available, can be combined with one or more phosphorescent materials emitting at a longer wavelength. Examples of phosphors include SrSi:Eu2+ and SrGaS4:Eu2+, which emit red and green light, respectively. However, the range of available phosphors limits the emission wavelengths that can be achieved using this method.
Quantum dot (QD) phosphors have been developed, which overcome some of the limitations of conventional phosphors. QDs, semiconductor nanoparticles of the order of 1-50 nm, can be tuned to emit at any wavelength from the UV to the near-IR region of the EM spectrum by controlling the particle size. Thus, simply manipulating the particle size during synthesis can control the colour of light emitted, even using a single type of QD material. Further, colloidally synthesized QDs are capped with organic ligands that impart solubility, making the materials solution processable. Combined with high fluorescence quantum yields, this means that tiny amounts of QD material are required to cover a large area.
In patent EP 1 259 412 B1 from Dialight Corporation, an LED lamp is proposed having one or more LEDs in a housing unit. By reverse mounting red and green LEDs, the circuitry can be arranged such that the application of a voltage of one polarity would result in emission from the red LEDs, while the application of the reverse polarity would result in green emission. While the lamp would result in superior longevity compared to incandescent lamps, requiring less frequent maintenance, and lower power consumption, the issue of colour blending is not address. Further, the circuitry required to produce multiple coloured emission would be more complex than that proposed in the present invention, where a single wavelength LED backlight can be used to produce both red and green (or indeed any other desired colour) emission.
Though LED-based signal heads do not fail as readily as incandescent lamps, one of their drawbacks is that current flow can continue even when an LED fails to emit. Further, failure of one LED typically increases the risk of concurrent failure of any other LEDs in the circuit. Consequently, systems to monitor and control the LED output have been developed. Patent application US 2005/0062481 A1 discloses an LED signal lamp with a data processor to monitor the output of each LED by matching its characteristics to a known diode curve. AlInGaP was proposed as a suitable yellow or red LED material, and InGaN as a green emitter. However, a problem with the system described in that application is that the green and the red/yellow emitting LEDs must be sorted (i.e., binned) to obtain lots that provide color consistency.
Patent GB 2472694 A from Unipart Rail Limited highlights the risks of using coloured LEDs, including inconsistent light output over the required operating temperature range for signal heads in the UK (−30 to 40° C.). Instead, it is proposed that more consistent light output can be achieved using white LEDs in combination with colour filters. Optionally, the colour filters may be shaped to act as lenses, and can be housed in a hooded signal unit to prevent undesired reflections known as “phantom signals”. However, colour filters absorb wavelengths of light that are not emitted, therefore there is a large amount of energy wastage.